1. Field of the Invention
The invention relates to an image pickup apparatus having two or more cameras capable of measuring a distance from an observer to a target and a rangefinder having such an image pickup apparatus.
2. Description of the Related Art
A rangefinder using a stereo camera is configured to take images of a same target from different visual points to measure a position of the target and a distance from an observer to the target based on a change in a distance between image forming points of the target in respective sensors. Recently, there is a growing concern in a driver's awareness of pedestrians in road safety and a driver's support system to attach a stereo camera on a vehicle to provide the driver information on existence of pedestrians in the vicinity of the driver's vehicle, and the like is about to be put in practice.
FIG. 18 illustrates a principle of a stereo camera system. The stereo camera system uses two cameras 31 and 32. In the stereo camera system, a distance Z from the observer to the target is computed based on the following equation:Z=B*f/d  (1)where a base length B indicates a distance between the camera 31 and the camera 32, a focus distance f is the camera focal length, a parallax d is the difference between image forming points in one camera obtained from different visual points, and the distance Z indicates a distance from the observer to the target.
However, the parallax of the stereo camera (parallax between the two cameras) decreases with an increase in the distance from the observer to the target, which may result in a large measurement error. When measuring a target at a long distance, accuracy in the distance measurement increases with an increase in the base length B between the two cameras. However, a viewing position from one camera to the target may greatly depart from a viewing position from the other camera to the target based on the positions of the two cameras as illustrated in FIGS. 19A and 19B, and hence searching time to determine the corresponding viewing positions to the target may take a long time. In addition, different aspects of the target are to be viewed from the corresponding camera positions, and hence it may be difficult to determine the corresponding viewing positions to the target. It is difficult to measure a whole distance range between the longest distance and the shortest distance of the observer to the target; however, various related art methods for measuring the whole distance range have been put forward such as a method for measuring the whole distance range using a combination of a stereo camera having a wide-baseline length and a stereo camera having a narrow-baseline length, or a method for measuring the whole distance range by determining the corresponding viewing positions with a narrow baseline length and individually moving the cameras to increase the baseline length to have the widest possible range for capturing the image.
Further, Japanese Patent Application Publication No. 11-39596 (hereinafter also referred to as “Patent Document 1”) discloses an external vehicle monitoring apparatus utilizing a short distance stereo camera and a long distance stereo camera. The external vehicle monitoring apparatus having the short distance stereo camera and the long distance stereo camera is capable of measuring a distance from a vehicle to a target in a range of 5 to 100 m by simultaneously operating the two stereo cameras to acquire distance information from the two stereo cameras, thereby detecting a target position at a short distance and a target position at a long distance by software computation. Similarly, Japanese Patent Application Publication No. 2008-129439 (hereinafter also referred to as “Patent Document 2”) discloses a multi-camera system configured to have four cameras in one case.
The related art multi-camera system utilizing the combination of the long baseline length stereo camera and the short baseline length stereo camera is capable of measuring a short distance range of 5 to 30 m and a long distance range of 20 to 100 m, which indicates the incapability of measuring a distance range between an ultra-short distance range in millimeters and a long distance range of around 100 m. Further, it is generally expensive to produce the multi-camera system having the combination of the long baseline length stereo camera and the short baseline length stereo camera.
Moreover, it is also difficult to measure a distance range between the ultra-short distance range in millimeters mm and the long distance range of around 100 m by individually moving the cameras to increase the baseline length. In this system, the computation load may be high while determining the corresponding viewing points by moving the cameras. Further, in this system, since the baseline length is increased to the widest possible range to allow both the cameras to capture the target, the baseline length may be extended unduly. Further, unnecessary measuring time may be consumed while moving the cameras in long distance ranges, and calibration of the cameras may be extremely difficult due to the mobility of the cameras.
In the technology disclosed in Patent Document 1, an external vehicle monitoring apparatus includes a short distance stereo camera and a long distance stereo camera to measure a distance from a vehicle to a target by simultaneously operating the two stereo cameras to acquire distance information from the two stereo cameras, and subsequently detect the target position at a short distance and the target position at a long distance by software computation. However, with this technology, it may be difficult to cover a distance range between the ultra-short distance range in millimeters and the long distance range of around 100 m and reduce manufacturing cost.
Note that Japanese Patent No. 4378434 (hereinafter also referred to as “Patent Document 3”) discloses a technology utilizing a compound-eye camera having a lens array including plural lenses aligned in a plane and a sensor. The disclosed technology utilizing such a compound-eye camera is capable of measuring an ultra-short distance range in millimeters. However, the compound-eye camera (one compound-eye camera) is incapable of measuring a long distance range despite the capability of measuring the ultra-short distance range in millimeters.